Pg 326 (338) - Q20
You want the X coordinate of point B which is the midpoint(since AB=BC). The trick is that you are starting from one over; so it is the midpoint in a line that has a length of 10. Therefore, it will have a value of 5 in that line, but the line started at the point with the coordinates of (1,4). So, the value of X has to be 6(you've got to add a 1 to that 5).